Opposing or aligned holes are sometimes required in hollow parts, such as for connecting mechanical fasteners therethrough. The inside of the part may be pressurized to assist a punch in producing a hole in the part. For example, in the hydroforming of parts from a hollow metal part, the hydroforming pressure is used to assist the punch in producing the hole in the part. This eliminates the need for a secondary operation such as drilling or laser cutting to form the hole in an internally unsupported region of the part.
In a typical punching operation for a hydroformed part, as the punch is advanced to engage the forward surface of the material, the rearward surface is supported by the pressurized fluid. Upon further advancement of the punch through the material to shear a slug, the pressurized fluid continues to bear upon the material to be removed as a slug, as well as upon adjacent material. The slug is sheared under the mechanical force applied to the material by the cutting edge of the punch and the force applied to the material adjacent the slug by the pressurized fluid.
The presence of a loose slug within the part poses several problems. In many instances, the presence of a loose or detached slug within the part may not be identified for some time, or even after the part has been installed in a finished product. Many systems have been developed to capture slugs formed by the punching operation. See, for example, U.S. Pat. No. 4,989,482 (Mason), issued Feb. 5, 1991, and assigned to the assignee of the present application. Slug capture is also an issue in applications where opposing holes are to be formed in the part. Examples of methods for obtaining slugs formed by such operations are described in U.S. Pat. No. 5,666,840 (Shah et al.), and in U.S. Pat. No. 6,067,830 (Klages et al.), issued May 30, 2000, and assigned to the assignee of the present application.